chore(submitqueue): remove dead pusher extension

## Summary

### Why?

The `pusher` extension is dead code. Merge execution moved to Runway, where the pluggable contract is `runway/extension/merger` — a Runway-owned interface that takes a `MergeRequest` off the queue rather than SubmitQueue `entity.Batch` values. Nothing in the tree imports `submitqueue/extension/pusher` any more: the only remaining references were the package's own files and the `mocks` target in the Makefile.

Removing it now keeps a second, divergent merge contract from drifting alongside the live one.

### What?

Deletes the whole `submitqueue/extension/pusher` tree — the `Pusher`/`Factory` contract, the git-backed implementation, the fake, and the generated mock.

Drops `./submitqueue/extension/pusher/...` from the `go generate` invocation in the `mocks` Makefile target. Left in place it would point `make mocks` at a path that no longer exists.

Drops the stale `pusher` mention from the `changeset` package docs, which listed it as an example of a decision/action extension resolving content through a `Resolver`.

## Test Plan

✅ `make gazelle` — no BUILD churn beyond the deleted packages
✅ `make fmt`
✅ `make test` — 77/77 pass
✅ `grep -rn "extension/pusher"` over the tree returns nothing

Author: behinddwalls

Makefile

@@ -366,7 +366,7 @@ local-stovepipe-stop: ## Stop the Stovepipe service
 
 mocks: ## Generate mock files using mockgen
 	@echo "Generating mocks..."
-	@$(BAZEL) run @rules_go//go -- generate ./submitqueue/extension/storage/... ./submitqueue/extension/buildrunner/... ./submitqueue/extension/changeprovider/... ./platform/extension/counter/... ./platform/extension/messagequeue/... ./submitqueue/extension/queueconfig/... ./submitqueue/extension/mergechecker/... ./submitqueue/extension/pusher/... ./submitqueue/extension/scorer/... ./submitqueue/extension/conflict/... ./submitqueue/extension/validator/... ./platform/consumer/... ./stovepipe/extension/storage/... ./stovepipe/extension/sourcecontrol/...
+	@$(BAZEL) run @rules_go//go -- generate ./submitqueue/extension/storage/... ./submitqueue/extension/buildrunner/... ./submitqueue/extension/changeprovider/... ./platform/extension/counter/... ./platform/extension/messagequeue/... ./submitqueue/extension/queueconfig/... ./submitqueue/extension/mergechecker/... ./submitqueue/extension/scorer/... ./submitqueue/extension/conflict/... ./submitqueue/extension/validator/... ./platform/consumer/... ./stovepipe/extension/storage/... ./stovepipe/extension/sourcecontrol/...
 	@echo "Mocks generated successfully!"
 
 proto: ## Generate protobuf files from .proto definitions

submitqueue/core/changeset/changeset.go

@@ -15,7 +15,7 @@
 // Package changeset resolves batch identity into the changes a batch contains.
 // It is the single place the orchestrator walks batch -> requests -> changes,
 // consolidating what the build and merge controllers each did privately.
-// Decision/action extensions (scorer, buildrunner, pusher, and future
+// Decision/action extensions (scorer, buildrunner, and future
 // detail-aware conflict analyzers) take thin identity entities and resolve their
 // granular content through an injected Resolver instead of being handed
 // pre-resolved data by a controller.